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Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa ), which is equivalent to 1,013.25 millibars , [ 1 ] 760 mm Hg , 29.9212 inches Hg , or 14.696 psi . [ 2 ]
Inch of mercury (inHg and ″Hg) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity. Conversion to metric units depends on the ...
Map showing atmospheric pressure in mbar or hPa A tire-pressure gauge displaying bar (outside) and pounds per square inch (inside) Atmospheric air pressure where standard atmospheric pressure is defined as 1013.25 mbar, 101.325 kPa, 1.01325 bar, which is about 14.7 pounds per square inch. Despite the millibar not being an SI unit ...
For example, if a barometer located at sea level and under fair weather conditions is moved to an altitude of 1,000 feet (305 m), about 1 inch of mercury (~35 hPa) must be added on to the reading. The barometer readings at the two locations should be the same if there are negligible changes in time, horizontal distance, and temperature.
In aviation, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.921 inches of mercury (1,013.2 mbar; 14.696 psi) as measured by a barometer. [2]
Pressure as a function of the height above the sea level. There are two equations for computing pressure as a function of height. The first equation is applicable to the atmospheric layers in which the temperature is assumed to vary with altitude at a non null lapse rate of : = [,, ()] ′, The second equation is applicable to the atmospheric layers in which the temperature is assumed not to ...
However, a common temperature and pressure in use by NIST for thermodynamic experiments is 298.15 K (25 °C, 77 °F) and 1 bar (14.5038 psi, 100 kPa). [ 4 ] [ 5 ] NIST also uses 15 °C (288.15 K, 59 °F) for the temperature compensation of refined petroleum products, despite noting that these two values are not exactly consistent with each other.
The variation in temperature that occurs from the highs of the day to the cool of nights is called diurnal temperature variation. Temperature ranges can also be based on periods of a month or a year. The size of ground-level atmospheric temperature ranges depends on several factors, such as: Average air temperature; Average humidity